This invention relates to separation of synthetic plastic materials and, more particularly, to processes for effecting such a separation by froth flotation.
Synthetic plastic materials, particularly long-chain, organic polymers and copolymers, are used for a variety of applications. It often is desirable to recover such plastic materials from refuse for recycling. When only one type of plastic material is present in the refuse, separation of the plastic material is relatively easy because the properties can be well controlled. However, when mixtures of two or more plastic materials are involved, separation of one type plastic material from the others is much more difficult because of a wide variation in the properties of the different plastic materials.
The approach most widely used for separating plastics from waste for recycling employs a heavy liquid or medium having a specific density. Plastic materials having a density higher than the medium sink and plastic materials having a lower density float. This approach is generally suitable for relatively pure polymers. However, it generally is ineffective for separations of waste containing a mixture of a number of different type plastic materials, coated plastic materials or plastic materials containing additives, for example, fillers, such as calcium carbonate, kaolin, silica, titanium oxide, colorants, antioxidants and the like. The densities of filled plastic materials can vary over a relatively wide range, depending on the type and concentration of the filler(s). Consequently, density separation generally cannot be used for such plastic materials.
It is known to use froth flotation to separate certain types of plastic materials. For example, Valdez et al. U.S. Pat. No. 4,167,477 discloses conditioning a mixture of polystyrene and two different polyamide plastics with sodium silicate and then subjecting the conditioned mixture to froth flotation to float polystyrene. The cell product from the first flotation is subjected to a second flotation, after being conditioned with a primary fatty amine, or salt thereof, to separate one polyamide plastic from the other. Sodium silicate acts as an inorganic wetting agent in the first flotation and the primary fatty amine acts as a cationic promoter to make one of the naturally unfloatable polyamide plastics, such as nylon, floatable. Use of a primary fatty amine as disclosed in this patent would be ineffective for separating a mixture of plastic materials normally having hydrophobic characteristics (i.e., are floatable), because none would remain in the cell product.
Izumi et al. U.S. Pat. 3,925,200, Izumi et al. U.S. Pat. No. 3,926,790 and Izumi et al. U.S. Pat. No. 3,926,791 disclose conditioning a mixture of plastic materials including polyvinyl chloride with an alkaline metal or alkaline earth metal salts of lignosulfonic acid or a hydrophilic organic colloid, such as tannic acid, quebarcho extract, gelatine, glue, saponin and the like, which act as a wetting agent to render the surfaces of polyvinyl chloride more hydrophilic and then subjecting the conditioned mixture to froth flotation to cause the polyvinyl chloride to separate with the cell product.
Saitoh et al. U.S. Pat. No. 3,985,650 discloses conditioning a mixture of a polyolefin, a polystyrene and ebonite with similar wetting agents to render the polystyrene and ebonite more hydrophilic and cause them to separate from a float fraction containing the polyolefin and then adding a frothing agent to the cell product from the first flotation to float polystyrene from ebonite.
Saitoh et al. U.S. Pat. No. 4,132,633 discloses conditioning a mixture of polyethylene, polypropylene and polystyrene with similar wetting agents to render the surfaces of the polystyrene more hydrophilic so that the polyethylene and/or polypropylene floats from the polystyrene and the polystyrene remains in the cell product.
The latter three flotation processes are limited with respect to the type of plastics which can be separated and do not provide an acceptable separation of some plastic materials.